(a) Field of the invention:
The present invention relates to a liquid crystal display device of a guest-host type using a mixture of a liquid crystal having a positive dielectric anisotropy and a pleochroic dye, and more particularly, it pertains to such guest-host type liquid crystal display device which is capable of making a positive display, i.e., a display of a colored image against a colorless background.
(b) Description of the prior art:
There has been proposed heretofore a large variety of liquid cyrstal display devices of the so-called guest-host host type which use a mixture of a liquid crystal material having a positive dielectric anisotropy serving as a host material and a pleochroic dye serving as a guest material, arranged in a display cell for the purpose of effecting a colored display. Most of these prior art display devices however, are of a negative type that a colorless image appears against a colored background. This is accomplished by the use of the aforementioned type of liquid crystal which has a large dielectric constant in the direction of the longitudinal axes of the liquid crystal molecules. In case such liquid crystal is used to construct a liquid crystal display device, it should be understood that, if the longitudinal axes of the molecules of the liquid crystal are not arranged to be oriented in a direction parallel to the planes of the two parallel glass substrates when no voltage is applied across the electrodes, the display device will be unable to effect a display of an image in response to a voltage applied across the electrodes which are designed to have a configuration corresponding to that of the pattern of the image to be displayed. In such device as mentioned above, an application of a voltage across the electrodes will cause the molecules disposed therebetween to assume an orientation perpendicular to the plane of the substrate. On the other hand, the longitudinal axes of the molecules of the dichroic dye also are oriented in the same direction as those of the molecules of the liquid crystal, irrespective of whether or not there is being applied a voltage across the electrodes. Thus, under no voltage applied, the entire front side of the display device will be visualized as presenting an allover colored display in the color of the dichroic dye as a result of its absorption of light. On the other hand, when a voltage is applied, a colorless image will appear due to the disappearance of the light-absorption effect exerted by the dichroic dye against a colored background provided on those portions other than those where the electrodes are provided.
Alternatively, the positive display, i.e., display of a colored image on a colorless background, may be realized by using a liquid crystal having a negative dielectric anisotropy and by arranging the molecules of the liquid crystal disposed between electrodes in a direction perpendicular to the parallel planes of the substrates when no voltage is applied, so that an application of a voltage will cause re-arrangement of the molecules to assume an orientation in a direction parallel to the planes of the substrates. However, there are some problems in the characteristic of the cell in terms of the level of the voltage required for driving the molecules in the cell, reorientation speed in response to the driving voltage applied and the serving life of the device. For these reasons, this positive display cell has not yet been placed into practical use.